This invention relates to improvements in the faceplate structure of the image pickup tube particularly of the type for use in a single pickup tube color television camera having a built-in color separation stripe-shaped filter (hereinafter simply referred to as "stripe filter").
Generally, in image pickup tubes for single pickup tube color television cameras capable of effecting color separation by means of a built-in stripe filter, the circuitry adopted therein is widely diverse, and in accordance therewith, the stripe filter and photoelectric signal take-out electrode in these image pickup tubes have various shapes and structures. Despite the difference in types of circuitry, the image pickup tubes of all types are always required to be constructed so that a stripe filter, a photoelectric conversion film and a transparent conductive film for take-out of photoelectric signal should be located optically in extremely close proximity to each other. In order that the optical proximity of the elements mentioned above may be realized in the faceplate, there have been heretofore adopted the structure in which the stripe filter is disposed on a fiber optic faceplate opposite to the photoelectric conversion film, the structure in which the stripe filter formed on one base and the photoelectric conversion film formed on the other base are joined to each other, and the structure in which the stripe filter is formed on a transparent base and, with a protective layer of an optically negligible thickness disposed thereon, the transparent conductive film and the photoelectric conversion film such as, for example, a photoelectric conductive film are overlaid thereon. The present invention aims to provide improvements in the faceplate of the last of the three structures mentioned above. The stripe filter comes in two types; the one type formed of an inorganic multi-layer dichroic filter and the other type formed of an organic light selective filter. The improvements provided by this invention are meant for the image pickup tube using the stripe filter of the latter type.
Generally, in the organic filter, thin-film stripes such as of gelatin are formed on a glass base and dyed with an organic dyestuff. Since the edges of the thin-film stripes on the organic filter generally are not sharp, the next upper layer can be formed immediately on the filter. When necessary, however, there may be used a modified type in which the thin-film stripes are coated with an organic film intended as a protective film and the next upper layer is formed on the protective film. In the image pickup tube which uses such an organic filter as described above, since the surface of the photoelectric conversion film is generally required to be exposed to a high degree of vacuum and to be scanned with an electron beam, it is necessary that the base supporting the photoelectric conversion film should be of such material and structure as to provide fully reliable performance under such high degree of vacuum.
To meet this requirement, therefore, an intermediate protective layer of glass is interposed between the surface of the organic stripe filter (or the surface of the organic protective film in the case of the filter which is provided with this organic protective film) and the photoelectric conversion member (which consists of a transparent conductive film and a photoelectric conversion film such as, for example, a photoconductive film) by bonding a very thin glass sheet to the opposed surfaces or by applying a glass plate to the organic stripe filter surface, subsequently grinding and/or polishing the glass plate to a small thickness, so that the intermediate protective glass layer may prevent the gas emitted by the organic filter from leaking into the high degree of vacuum mentioned above.
As the transparent conductive film to be applied to the faceplate formed with the organic stripe filter, there is frequently used a cold formed transparent conductive film which can be formed on the intermediate protective glass layer, subsequently to the formation of this protective layer, at working temperature and time within an allowable heat resisting limit condition for the organic stripe filter. The coat which is composed preponderantly of In.sub.2 O.sub.3 and formed by sputtering process is one typical example. If the transparent conductive film is prepared in the form of a non-porous film of from 500 to 5000A thickness on the organic stripe filter without degrading the optical transmittivity required of this conductive film, this film suffers from the structural defects common to the non-porous film, i.e. heavy occurrence of pinholes in the direction of the film thickness. With this defective film, therefore, the gas emitted by the organic filter cannot be thoroughly prevented from leaking into the aforementioned high degree of vacuum. As the photoelectric conversion film, the film which is prepared by vacuum depositing such a principal constituent as Sb.sub.2 S.sub.3, PbO or Se on the transparent conductive film is adopted in the image pickup tube of the photoconduction type. The material for this photoconductive film is widely variable with the kind of the film, and the method of vacuum deposition is also variable with the material to be adopted. Since mere lamination of the aforementioned transparent conductive film and photoelectric conversion film does not warrant preclusion of the gas leakage, the interposition of the intermediate protective glass layer described above has been found indispensable.